He has asked the chairman of the new Independent Safeguarding Authority to review this and report by December.

The Tories said the plan was still too "vague" amid fears it could affect the running of sport and other clubs.

Mr Balls stressed the rules would not apply where, for example, parents agreed to give friends' children "a lift to school or to Cubs".

"Nor will it cover instances where parents work with children at school or a youth club on 'an occasional or one-off basis'," he said in a letter to the chairman of the Commons children, schools and families committee, Barry Sheerman.

Shadow Home Secretary Chris Grayling said: "All of this is so vague that in reality clubs and other organisations up and down the country will act to be on the safe side.

"So they'll register all of the parents who are involved even loosely. So the result is we'll get this huge expensive and cumbersome bureaucracy as well as volunteers giving up."

Lib Dem home affairs spokesman Chris Huhne welcomed the review as the plans "were so disproportionate that they were going to put off masses of people from volunteering".

Children's charity the NSPCC said new procedures were needed but called on the government to provide more detailed information about how the scheme would work.

Chief executive Andrew Flanagan said: "People want to make sure children are protected but need to fully understand and buy into any major new plan that helps to do so.

"This review and improved information about how it works will hopefully allay confusion and misunderstandings about what the scheme is meant to do and lead to its successful introduction."

'Misleading'

Mr Balls said the scheme had been introduced via legislation in parliament, following the Bichard inquiry into the murders of two schoolgirls in Soham.

It requires those working with children or with vulnerable adults, either on a paid or voluntary basis, to be on a register of suitability which employers can check.

The system will be phased in from next month and will operate in England, Wales and Northern Ireland from next year. A different scheme is being introduced in Scotland.

Mr Balls said there had generally been very strong support for the scheme during extensive consultation.

"Recently, however, some concerns have been expressed about the precise interpretation of a particular aspect of the scheme; that is, the degree of contact with children which should trigger the requirement to register," he added.

There had been "some inaccurate and misleading reports".

But striking the right balance had been a difficult judgement, in particular how precisely the "frequent or intensive" principle should be applied to real life situations.

Ministers had asked Independent Safeguarding Authority (ISA) chief Sir Roger Singleton "to check the government has drawn the line in the right place on this issue".

Sir Roger said he would look carefully at the criticisms of the scheme.

But he said he did not want parents going about their "normal business" being inconvenienced by additional measures.

Agencies

Children's authors including Philip Pullman and Michael Morpurgo had complained the requirement was "insulting" and said they would stop visiting schools.

But this is not the only apparent concern being raised.

Anne Fairweather, head of public policy at the Recruitment and Employment Confederation, said the vetting and barring scheme, once fully operational, would bring significant benefits.

"However, we have serious concerns about the way the scheme is being introduced.

"We are less than a month away and it is still not clear what information recruitment agencies should refer to the Independent Safeguarding Authority and when this should take place."